The release behavior of the fission products iodine and cesium has been characterized in fission product release tests that have been conducted at Oak Ridge National Laboratory with highly irradiated, light water reactor fuel segments under conditions simulating severe accidents. The chemical forms of the fission products depended on the composition of the carrier gases used in the tests. In purified helium or steam-helium-hydrogen atmospheres, the behavior of the released iodine was characteristic of cesium iodide, whereas that of the cesium, which was not associated with cesium iodide, was characteristic of cesium oxide in the helium atmosphere and of cesium hydroxide in the steam-helium-hydrogen atmosphere. In the dry-air tests, iodine appeared to be in elemental form and cesium in the oxide form. In the steam-helium-hydrogen tests, the released cesium (other than CsI) significantly reacted with and was retained by hot oxidized stainless steel, zirconia, and silica surfaces. In contrast, cesium iodide appeared to be unaffected by these surfaces.